Fibre Channel is an American National Standards Institute (ANSI) set of standards which describes a high performance serial transmission protocol which supports higher level storage and networking protocols such as HIPPI, IPI, SCSI, IP, ATM, FDDI and others. Fibre Channel was created to merge the advantages of channel technology with network technology to create a new I/O interface which meets the requirements of both channel and network users. Channel technology is usually implemented by I/O systems in a closed, structured and predictable environment where network technology usually refers to an open, unstructured and unpredictable environment.
Advantages of Fibre Channel include the following. First, it achieves high performance, which is a critical in opening the bandwidth limitations of current computer to storage and computer to computer interfaces at gigabit speeds. Second, utilizing fiber optic technology, Fibre Channel can overcome traditional I/O channel distance limitations and interconnect devices over distances of 6 miles at gigabit speeds. Third, it is high level protocol independent, enabling Fibre Channel to transport a wide variety of protocols over the same media. Fourth, Fibre Channel uses fiber optic technology which has very low noise properties. Finally, cabling is simple in that Fibre Channel typically replaces bulky copper cables with small lightweight fiber optic cables.
Fibre Channel supports three different topologies, point-to-point, arbitrated loop and fabric attached. The point-to-point topology attaches two devices directly. The arbitrated loop topology attaches devices in a loop. The fabric attached topology attaches a device directly to a fabric.
In a Fibre Channel Arbitrated Loop topology up to 126 devices and one connection to a fabric switch may exist in a single Arbitrated Loop Physical Address (ALPA) space. Data within an ALPA space physically travels from node to node in a daisy-chain fashion, ultimately traveling in a loop. Control by a device on the loop is obtained through the process of loop arbitration, after which the device winning arbitration sends data. In a half duplex mode, only one device is transferring data at any point in time. In a full duplex mode, two devices may communicate with each other at the same time.
Generally, the disadvantages of the Arbitrated Loop topology include: first, it is a blocking topology, that is, only a single connection between a pair of nodes is allowed at any point in time (excluding the broadcast mode). Second, device buffering occurs in each device as it has a six word buffer, creating a delay of up to 225 nanoseconds. This delay is additive with each device in the loop. The delay creates overhead for the communicating devices when a large number of devices are connected to a loop. Third, distance also adds delay to a loop and is additive for each device. Fourth, robustness is an issue since all devices are on one loop any device failure will cause the entire loop to fail or reset. Fifth, the total bandwidth available is limited to the bandwidth of the loop itself.
Loop devices are typically interconnected on an Arbitrated Loop with a hub. The hub is a passive device, that is a loop exists within the hub. A hub in most cases maintains the loop's integrity when devices are removed, powered off, or fail by using port bypass circuits. Hubs simply receive and redrive the signals to individual devices.
Hub advantages include low cost, low complexity, ease of use and interoperable with a large number of Fibre Channel Arbitrated Loop supported devices.
There are also many disadvantages when interconnecting Fibre Channel Arbitrated Loop devices with hubs: First, hubs do not address the blocking nature of the loop topology. Second, jitter is propagated from bypassed nodes. This additive affect causes loop instability when a large number of devices are interconnected. Third, when data is currently being transferred and a device attached to a hub is powered off or fails, the loop could be reset which is destructive to the communicating devices. Fourth, if a device is inserted into a live loop the loop will be reset which is destructive to the communicating devices.
In Berman U.S. Pat. No. 6,185,203, entitled “Fibre Channel Switching Fabric”, there is disclosed apparatus which comprises separate port control modules, one for each attached device, a central router module, a switch core module, a fabric control module and a brouter (bridge/router) module. The port control modules are connected to the router modules by separate route request connections and separate route response connections. Through this structure, route requests may be provided from the port control module to the router while simultaneously the router provides route request responses to the same port control module. Preferably, a common route request channel is utilized. Thus, apparatus is provided to return a route response to a previously requesting port while other ports are arbitrating and sending route requests to the centralized router. More generally, this apparatus provides for reading resource requests from multiple requesters while at the same time returning resource grant responses to previous requesters.
In Berman U.S. Pat. No. 6,118,776, entitled “Methods And Apparatus For Fiber Channel Interconnection Of Private Loop Devices”, methods and apparatus are provided for Fiber Channel interconnection between a plurality of private loop devices through a Fiber Channel private loop device interconnect system. In the preferred embodiments, the Fiber Channel private loop device interconnect system is a fabric or an intelligent bridging hub. In one aspect, a Fiber Channel private loop device is connected to two or more Arbitrated Loops containing, or adapted to contain, one or more private loop devices. Preferably, the interconnect system includes a routing filter to filter incoming Arbitrated Loop physical addresses (ALPAs) to determine which Fiber Channel frames must attempt to be routed through the fabric. Numerous topologies of interconnect systems may be achieved. In another aspect, a method is provided for implementing a logical loop of private loop devices by switching the logical loop into a plurality of sets, assigning each set to a physical Arbitrated Loop and connecting the Arbitrated Loops to a Fiber Channel private loop device interconnect system. Additional methods are provided for restricting attached devices to Arbitrated Loop physical addresses within certain ranges. Additionally, methods are provided for resetting hosts, the method generally comprising the steps of detecting the addition of a storage device to a first Arbitrated Loop, and thereafter, resetting the Arbitrated Loop or loops on which a host or hosts reside on second Arbitrated Loop. Methods for operation with use of SCSI initiators generate a link service reject when no address match is found, or when an address match is found, but where no device with the destination ALPA exists on the Arbitrated Loop corresponding to the destination.
What is needed is an ability to interconnect Fibre Channel Arbitrated Loop devices in a low cost and simple to deploy manner.
As such, it is the goal of this invention to provide apparatus and methods that solves or mitigates these problems with a resulting solution that is low cost and easy to deploy and manage.